The invention relates to the photorefractive effect in glass. In particular, the invention relates to the application of the photorefractive effect to the fabrication of optical devices based on chalcogenide glasses.
The Fiber Optic Faceplate (FOFP) is an important product, finding applications in various technologies, e.g., night vision devices and CRT displays. FOFPs are manufactured by heating and drawing a core/clad preform into thin canes, bundling the canes, and redrawing them to yield a multifiber bundle. After several such redraws, the thin bundles, each consisting of many individual optical fibers, are heated and fused into a block. The block is cut and polished to yield a thin plate, cylinder, or other configuration which can serve as an image transfer device. In effect, the finished FOFP is a fused array of many thousands of very small optical fibers. The process is time consuming and labor and materials intensive.
The FOFP and other fused fiber devices made by this process are designed to operate at light wavelengths in and around the visible portion of the spectrum, where the glasses used to make them are transparent. In particular, infrared (IR) transparent optical fibers derived from materials known as chalcogenide glasses have emerged as important new materials having a variety of desirable applications.
The term chalcogenide glasses defines a large family of vitreous materials fabricated from metals (e.g., As, Ge, Sb) in conjunction with the heavier elements in the oxygen family (i.e., the calcogens S, Se, Te). There are literally hundreds of such glass-forming chalcogenide compositions; one well studied example is the glass known as arsenic trisulfide or As.sub.2 S.sub.3. Generally speaking, chalcogenide glasses have low glass transition temperatures (typically 180.degree.-300.degree. C.) and high refractive indices (typically 2.5). While dependent on composition, the transparency range of these glasses spans (roughly) the 0.8 to 15 micron region.
Optical fibers drawn from chalcogenide glasses are known. It is also known to draw and fuse many such fibers into a bundle and use them to transmit simple IR images. There are problems, however, in maintaining the circular cross-section of the individual fiber elements, which tend to distort during the fusing process.
In another approach to making an IR image transmitting device, a thin metal plate containing an array of many small holes (made via lithography and etching) is dipped into a molten chalcogenide glass and then removed. The glass coats the plate and fills the holes. The plate is then polished to remove the surface glass. The result is an IR faceplate in which each glass-filled hole acts a like a fiber element. This technique is difficult to implement and is expensive. Moreover, the faceplate has limited applications.
In many chalcogenide glasses, the refractive index of the material may be increased by illuminating the material with light of an appropriate wavelength and intensity. This light-induced refractive index increase is referred to in the art by a variety of names such as photorefractive effect; photostructural transformation; photodarkening or photoinduced refractive index change (PRC). For simplicity, the designation PRC shall be used in this disclosure to describe the effect. Various PRC effects are summarized below.
It is important to note that the vast majority of PRC related results have been derived from experiments on chalcogenide glass thin films whose thickness is typically 1 to 10 microns. These are often prepared by laborious processes employing thermal evaporation or sputtering from crushed glass or bulk glass targets (discs) in a high vacuum system.
The physical origins of PRC are not well understood and are the subject of ongoing theoretical discussions in the literature. It is generally agreed, however, that the effect occurs in many chalcogenide glasses with the largest effect occurring in vitreous As.sub.2 S.sub.3 i.e., this glass exhibits the largest increase in index upon irradiation with light. The applications outlined in this disclosure are not limited to a particular chalcogenide composition, but arsenic trisulfide, As.sub.2 S.sub.3, will be used as an example.
In FIG. 1 curve A generally shows the estimated average of the absorption coefficient of As.sub.2 S.sub.3 within a range of values represented by the shaded area about the curve as a function of wavelength (or photon energy) in the visible portion of the spectrum. The data from which curve A was prepared is available from standard references, journal papers and measurements on glasses made by the applicant. There is generally good agreement among these various sources. At high values of the absorption coefficient (e.g., &gt;1 cm.sup.-1) the material is considered opaque. At lower values it is progressively more transparent in the longer wave lengths, &gt;0.6 .mu.m. The arsenic trisulfide thus appears red in color. Over much of the range of FIG. 1, the logarithm of the absorption coefficient is linear with wavelength as is apparent from the curve. Above about 0.6 microns, curve A bends. The origin of this "bend" is still debated, with some authors believing it due to iron impurities in the glass while others suggest that it is intrinsic (inherent) to the glass structure. Curve B is a plot of the measured absortion coefficient for a Ge-Sb-Se glass which is shifted by about 0.1 .mu.m towards the longer wavelengths.
The band-gap of the glass shown in FIG. 1 is loosely defined as the wavelength region where the absorption coefficient has a value of 10.sup.3 cm.sup.-1. The art refers to wavelengths shorter than this as band-gap light. Such wavelengths are almost entirely absorbed before light can progress more than a few microns into the material. Light of wavelengths longer than the band-gap is referred to as sub-band-gap light. Depending on wavelength, such light can penetrate millimeters to centimeters into the material without suffering a significant decrease in intensity.
There are several PRC effects which can occur when a chalcogenide glass is illuminated with light in the wavelength region spanned by FIG. 1. All such PRC effects produce an increase in the refractive index. These effects are referred to in the art by a variety of (partially self-explanatory) names: irreversible PRC; reversible PRC; dynamical PRC and transitory PRC. Which of these effects is operative depends on the wavelength of illumination, the prior thermal history and/or the illumination history of the glass sample.
For clarity, it should be noted that irreversible PRC occurs only in freshly deposited thin films whose structure is much different from that of bulk glasses made by melting and casting. When such fresh films are exposed to band-gap light, their structure photopolymerizes or densifies resulting in a large permanent increase in the refractive index, which cannot be reversed by heating. The same effect can be produced by heating an unexposed freshly deposited thin film. Heating causes densification and an increase in refractive index. Such a heat-treated or annealed film has an index (and other properties) which is considered to be representative of the bulk glass. These annealed films are often used as a starting point for experiments on other PRC effects.
Dynamical and transitory PRC phenomena are less permanent and/or are of smaller magnitude than the thermally reversible effects. They are of interest because they are switchable in one of two ways. In dynamical effects, illumination with one wavelength can increase the index slightly. Illumination with a different wavelength decreases the index to its original value. A transitory effect occurs upon illumination with band-gap light whereby the index increases during illumination but relaxes (decreases) slightly to a lower, but permanent, value when the light is turned off.
In considering the prior art it must be reemphasized that, the application of PRC effects has been limited to very thin chalcogenide glass films or platelets and employs comparatively short wavelengths in the region marked band-gap in FIG. 1. While the term bulk glass is occasionally used in certain references in the art, careful analysis of such references shows the term to have a subtle and often confusing meaning. For example, what appear to be small reversible PRC effects in bulk glasses were obtained when slices from a large ingot (made by conventional melting) were cut and polished to thickness of 10-30 microns for illumination with band-gap region light. Others have looked for reversible PRC in annealed thin films and compared their behavior with that of bulk ingots which were never illuminated. The implicit presumption is that the annealed film properties are representative of bulk glasses behavior. These results conclude that, there is no measurable change (.DELTA.n&lt;0.01) in the refractive index which accompanies illumination of such annealed films. A further example is contained in work which attempts to compare the reversible PRC behavior of film and so called bulk samples. Unfortunately, no information is available about the preparative conditions or size of the specimens. The work shows that irreversible index changes may occur in bulk samples, but only at cryogenic temperatures. Heating to room temperature eliminates the effect.
Despite the apparent confusion, some practical applications of PRC effects have been reported in the art. One important example is the use of PRC to store holographic images in thin films of chalcogenide glass. Although the glass layers used are only about 10-50 microns thick these have been referred to as volume or 3-dimensional holograms. The films are prepared either by evaporation or by squeezing a molten glass to minimal thickness between two cold plates. In these applications, the glass film serves as photographic medium. The holographic interference pattern is stored in the film as localized increases in the refractive index. The image can be erased or bleached either by heating or by uniformly exposing the plate to light of an appropriate wavelength. The latter has the effect of increasing the index of previously unexposed regions of the plate, such that the stored information, in effect, fades into the background. Such light-induced bleaching is an important concern.
The use of transitory and dynamical PRC effects to fabricate thin film optical signal processing devices has been investigated. These include switches, modulators, and light deflectors in which the propagation of one light beam is controlled by another via local (and temporary) changes in refractive index.
Reversible PRC effects have also been employed to write optical waveguides into evaporated thin glass films. A focused argon laser near 0.514 microns or an electron beam is scanned across the surface of a glass thin film. The beam creates a channel of increased refractive index in the surrounding glass whose width and depth is dependent on the beam spot size and intensity. The channel is thus a waveguide or optical fiber embedded in the film. The technique offers the prospect for fabricating thin film optical integrated circuits containing a variety of devices such as fiber couplers and signal splitters.
Chalcogenide thin films have also found application to the field of optical recording discs. This technology, however, makes use of optically induced (and reversible) crystallization in such glassy films.
Others have described applications for PRC effects in certain bulk plastics in which a cylindrical filament of high refractive index may be optically written deep within the body of a plastic cube by exposing the region to high intensity ultraviolet light. The end use appears to be optical integrated circuits in 3-dimensional bulk form, as opposed to the basically 2-dimensional thin film application outlined above. The concepts have not been extended to multiple filament applications or to glasses.
It would be desirable to have a method which could produce a device consisting of an array of many hundreds or thousands of individual fibers or fiber like structures which could be prepared without use of the draw/redraw/fuse or other mechanical approaches. It would also be desirable that the device operate at wavelengths in the infrared (IR) which is difficult to access with current technology. IR wavelengths of 3-11 microns, are of special interest as many imaging, thermal detection, and analytic systems operate in this region.